Forget about generation ships, suspended animation, or the sudden appearance of a worm hole. The most likely way for aliens to visit us — whatever their motive — is by sending robotic probes. Here's how swarms of self-replicating spacecraft could someday rule the galaxy.

Mathematically Efficient

The strength of Von Neumann's idea lies in the brute efficiency of exponential growth. Given enough time and patience, a single self-replicating probe (SRP) could produce millions upon millions of offspring; it would be like a massive bubble expanding outward into the Galaxy. Theoretically, these probes could occupy all four corners of the Milky Way in as little as half a million years – even if each probe were to travel at an average cruising speed of one tenth the speed of light (though, as I'll describe later, more recent estimates place it at 10 million years, which is still an incredibly short amount of time cosmologically speaking).

A Von Neumann probe is a robot designed to reach distant star systems and create factories which will reproduce copies themselves by the thousands. A dead moon rather than a planet makes the ideal destination for Von Neumann probes, since they can easily land and take off from these moons, and also because these moons have no erosion. These probes would live off the land, using naturally occurring deposits of iron, nickel, etc. to create the raw ingredients to build a robot factory. They would create thousands of copies of themselves, which would then scatter and search for other star systems.

In order to work, a von Neumann spacecraft would have to tap into advanced nanotechnology and artificial intelligence — technologies that advanced extraterrestrial civilizations are likely to develop. In fact, the device itself would be a molecular assembler, capable of reconstituting matter into copies of itself, which is why SRPs are also referred to as kinematic self-replicating machines.

And indeed, these probes would be remarkably efficient. A recent study published in the International Journal of Astronomy pointed out that extraterrestrial intelligences (ETIs) could use the slingshot effect to propel SRPs from star to star. And yes, that's the same method used to propel the Voyager spacecrafts through our solar system from planet to planet. For it to work on a galactic scale, however, SRPs would use slingshot maneuvers around stars, gaining a boost in velocity by extracting energy from each star's motion around the galactic center. The slingshot effect would carry little-to-no extra cost and result in a 100-fold increase in efficiency; models show that this technique could be used to send probes to every solar system in the galaxy in as little as 10 million years! Adding to the efficiency is the realization that SRPs could replicate on the fly, building duplicates of themselves while they're traveling. The probes would collect matter, like dust and gas, from the interstellar medium as they traverse vast distances.

A number of scientists, futurists, and sci-fi writers have speculated over the years about the different kinds of probes ETIs may wish to construct once they're ready to explore — or conquer — space in this fashion. Here's how they'll work.

Exploration and Reconnaissance Probes

We know that exploratory probes exist because we've created some ourselves, namely Voyager 1 and 2 — though strictly speaking they are not von Neumann replicators.

The Voyager spacecraft. Credit: NASA/JPL.

Exploration probes would be designed strictly for space exploration and surveillance (some of it even covertly); these autonomous devices would not contact or interact with other intelligent civilizations. Exploration probes could remain local to a solar system (so-called Astrochicken probes), or they could be sent on interstellar missions to explore and transmit their findings back to the home planet. These SRPs could study foreign solar systems in exquisite detail — and even alert the folks back home about the presence of extraterrestrial life.

These probes could also act as stationary reconnaissance stations. SRPs could take residence in a data rich area and continuously beam that information back to the home planet — and all without ever being detected.

And being von Neumann probes, they could spread exponentially. Of course, given that radio waves can only travel at the speed of light, and that we're talking about cosmic-scale distances, the time it would take for a signal to reach home would not be insignificant.

Communication probes (a.k.a. Bracewell probes)

The current SETI strategy of targeting stars and listening for radio signals has an extremely slim chance of success. It's a needle-in-the-haystack approach. That said, given the assumption that civilizations want to communicate with us, a more efficient way for them to make contact would be to disseminate self-replicating communication probes across the Galaxy.

Hypothetical Bracewell probe. Image via David Darling.

Dubbed Bracewell probes (named after Ronald N. Bracewell who thought of the idea back in 1960), these devices would work as an alternative to interstellar radio communication between widely separated civilizations. This strategy only makes sense given the inefficiency and weakness of radio signals emitted from the source planet. What's more, given the vast distances involved, these probes could contain pre-recorded — but decipherable — messages for us to decode (like a simple "hello!" or even a super-sophisticated stream of important information). We just need to be careful that it's not a trojan horse of some sort.

Multiple Bracewell probes could also be set up as a distributed array of communication relay stations. Such a set-up was portrayed in Carl Sagan's Contact. In this story, a dormant Bracewell probe was lying in wait in the Vega system. It began to transmit a strong signal after it received a radio signal from Earth. The device itself was part of a larger network of probes, as witnessed later by Ellie's journey from probe to probe.

Worker Probes

If aliens are going to embark on megascale engineering projects, they're going to need robots. Lots of 'em. Projects like Dyson Spheres, Ringworlds and Alderson Disks would require fleets of specialized and artificially intelligent probes working in concert to construct these truly massive structures.

Hypothetical Dyson sphere. Image: Eburacum45.

Given the sheer scale of these projects and the amount of matter that would have to be subverted, it's not unreasonable to assume that millions of individual probes would be required. The most sensible way to construct and disseminate these probes would be through self-replication schemes.

Indeed, as Oxford physicist Stuart Armstrong has noted, if we're going to do this ourselves at some future point, we'll have to resort to some extreme measures. Megascale structures will require a horrendous amount of material. A Dyson sphere would require so much matter that, should we want to completely envelope the sun, we would have to disassemble Mercury, Venus, some of the outer planets, and any nearby asteroids.

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So, SRPs could be put to work as mining machines that dig-out and transport matter across vast distances. Ideally, these probes would be programmed to work together and take advantage of swarming intelligence and emergent properties.

Colonization Probes

As noted, molecular assembling nanotechnology will make it possible for probes to go about interstellar colonization. It's conceivable that an SRP could find a suitable planet and use the matter around it to not just reproduce itself, but to establish a colony and seed actual settlers.

Such settlers would likely be uploaded consciousness patterns. This would obviously require an incredibly sophisticated mind emulation scheme, powerful artificial intelligence, and advanced supercomputing. Ideally, these consciousness patterns would be able to migrate to a robotic body for corporeal investigation of the environment. The number of settlers in any given location could be significant, limited only by computational resources.

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Colonization probes, sometimes referred to as seeder probes, could also perform double-duty as terraformers. Project Genesis, as portrayed in the Star Trek film series, utilized such a probe, which was able to transform a dead planet into one that suited the needs of its future inhabitants.

Uplift probes

Probes could also work to transform and "uplift" other civilizations and their citizens. This scenario was explored in 2001: A Space Odyssey in which an advanced extraterrestrial civilization used probes (called monoliths) to steer the direction of evolution on Earth. In the story, these probes endowed primates with the capacity to use tools, and later, the human David Bowman was transformed into the next stage of evolution, the so-called Star Child.

This scenario was also explored in David Brin's Uplift series in which advanced civilizations brought sapience to primitive life forms — what's more accurately termed biological uplift. Also conceivable is technological or civilizational uplift in which an extraterrestrial intelligence brings an entire civilization up to its own advanced level.

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Motivations for doing so could involve meta-ethical imperatives meant to reduce suffering, to prevent civilizations from destroying themselves, or to ensure the safe onset of non-threatening post-Singularity intelligences. Or, it could be part of their plan to take over the Galaxy.

Uplift probes could quickly bring a civilization to a post-Singularity, postbiological condition. Such a force might appear as a colonization wave that sweeps across the Galaxy, transforming all that it touches into computronium. Such a scenario has been projected by such thinkers as Hans Moravec and Ray Kurzweil.

Berserker probes

Unfortunately, we need to be on the lookout for malevolent probes, what Fred Saberhagen dubbed Berserkers. Just as an intelligent civilization could use self-replicating probes to spread life across the Galaxy, another misguided or evil civilization could do quite the opposite and destroy everything.

Berserkers could also be mutated SRPs that are running amok. To prevent this, responsible ETIs should implement failsafes that immediately shut-down replication in the event of data corruption. It's possible, however, that a random, but wide-ranging scrambling of codes (a macro-mutation to use the parlance of evolutionary biology), could override such measures.

Berserking SRPs could be disseminated with the sole purpose of sterilizing every planetary system it encounters, forever eliminating the possibility for life to emerge and evolve. Should it encounter an inhabited planet, it could use any number of schemes, including nanotech instigated ecophagy, to quickly destroy all life in a matter of hours. By using a scorched galaxy policy, a civilization could sterilize the Milky Way in as little as 10 million years.

Alternately, berserker probes could be disbursed across the entire Galaxy and lie dormant, patiently waiting for signs of intelligence.

Berserkers could also work to stamp out intelligent life that it deems dangerous. In one scenario, an advanced civilization (or Galactic club) could monitor for potentially dangerous post-Singularity artificial superintelligences and quickly stamp them out of existence.

It's worth noting that Earth has not been sterilized by a berserker probe, a possible sign that they don't exist, or that they haven't destroyed us yet.

Police Probes

It's not unreasonable to suggest that probe-making civilizations would also be thinking about defensive measures. Futurist Anders Sandberg has devised an idea for anti-berserker policing probes — devices that would be on the lookout for malevolent SPRs of any kind and take action.

Civilizations might want to establish quarantined areas; policing probes would ensure that nothing gets through the defenses and ensure the integrity of a specified region. Xenophobic civilizations might want to set up quarantined areas to prevent memetic infection, to protect themselves against invasions or intrusions, or simply due to a fear of the unknown.

The best way of stopping a replicator, argues Sandberg, is to nip it in the bud. To do so, an advanced civilization would require widespread surveillance and enough power to deal with possible threats. And because replicators could emerge outside a given region of control, a civilization would want to have widely stockpiled defenses. The easiest way of doing this? Yup, you guessed it: make a replicator that spreads and builds these stockpiles and quietly waits for signs of something threatening.

So, where are all the probes?

Given all this technological potential, one must wonder why we haven't encountered any extraterrestrial probes. Why haven't extraterrestrials communicated with us? Why haven't we be uplifted....or destroyed?

This conundrum was first articulated by Frank Tipler and has become a critical driver of the Fermi Paradox. It's been a cause of much the contact pessimism that has taken root since the 1970s (my own inclinations included). If it's so easy for probes to colonize the Galaxy, then where the heck are they? Tipler concluded that extraterrestrials simply don't exist.

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Carl Sagan and William Newman came up with a different answer. They were convinced that Tipler had it all wrong and that all this talk of probes was sheer poppycock. In their 1983 paper, "The Solipsist Approach to Extraterrestrial Intelligence," they calculated that von Neumann probes, should they exist, would eventually start to consume most of the mass in the Galaxy. They concluded that intelligent civilizations would never dare construct such probes and would try to destroy any such device as soon as it was detected.

Indeed, Sagan and Newman's argument isn't convincing. Probes with even a modicum of AI and smart programming could be programmed to stop after a certain reproductive threshold has been achieved (time-to-produce schemes, maximum number of iterations, etc.). These probes wouldn't be simple mindless automatons. Moreover, the Sagan and Newman theory violates non-exclusivity; it might explain why most civilizations wouldn't dare embark on such colonization schemes, but not all. All it would take is just one.

And as Sandberg has since noted, it'll take a fleet of SRPs to counter SRPs. What's more, as he noted to me:

One of the interesting things with police probes is that it makes strategic sense to announce that they are around to civilizations that might "break the law" — yet not reveal exactly how strong they are or what their modus operandi is.

Further, says Sandberg, one species' police is another species' invader — we would probably not like having some alien probe impose their view of what is an unacceptable activity on us, and vice versa. And the process of making police probes will likely be indistinguishable from making other replicators. Consequently, there might be a race to set up the first interstellar police force.

At any rate, the reason for the absence of any kind of probes remains a mystery.